1. Field of the Invention
The present invention relates to water soluble colorants and, more particularly, to water soluble colorants for ink jet printing having improved water fastness on paper.
2. Background Information
In ink jet systems, highly water soluble colorants have advantages over pigment dispersions, such as better ink stability and fewer problems associates with jet clogging. However, conventional colorants often produce prints with lower water resistance on paper. Another problem is that ink formulations based on some conventional water soluble dyes may “bleed,” wherein two colors printed side by side spread into each other before the ink has dried resulting in ill-defined borders.
Various methods have been proposed for improving the water resistance of water soluble dyes. For example, EPA 89-308157 discloses the conversion of carboxylic acid dyes, in their salt form, to the less soluble acid form on acidic paper. Zwitterionic dyes, whose molecules contain both strongly basic and acid groups, have also been proposed to improve water resistance. See WO 94-GB2637.
U.S. Pat. No. 5,919,846 discloses a colorant, which is the addition product of an organic chromophore having at least one reactive hydroxyl or amine substituent, a polyisocyanate, and a carboxylic acid, sulfonic acid, or salt of either thereof.
Despite previous work, there is a continuing need for water soluble colorants for ink jet printing having improved properties, such as water fastness, and for ink jet formulations employing such improved colorants.